The present invention relates to an electric article, particularly to an electric cable or an accessory thereof, such as a cable joint or a cable termination, comprising at least one element made from a semiconductive polymeric material, and to a semiconductive polymeric composition.
Electric cables, particularly electric cables for medium or high voltage, usually comprise at least one electric conductor, at least one insulating layer and at least one semiconductive layer. Particularly, a first semiconductive layer is usually placed between the electric conductor and the insulating layer, while a second semiconductive layer is applied in contact with the external surface of the insulating layer. In some applications, the cable is further equipped with at least one metallic shield placed in a radially external position with respect to the second semiconductive layer. The semiconductive layers operate to provide uniform electric field around the cable insulation by reducing the potential gradient over the surface of the stranded conductors and inside the metal shielding, and to prevent corona discharge at the surfaces of the stranded conductors and the insulation. Moreover, the semiconductive layers should protect the cable against damage caused by conductors heating due to short-circuited current.
Elements made from semiconductive polymeric materials are also used in electric cable accessories, particularly in cable joints and cable terminations, where it is essential, in order to prevent electric breakdown, to provide the accessories with such elements placed in correspondence with the zones where cable insulating and/or semiconductive layers are interrupted so as to avoid excessive concentrations of flux lines of the electric field.
Elements, and particularly layers, made from semiconductive polymeric materials are usually produced by extrusion of polymeric compositions containing at least one carbon black as electrically conductive filler. The conductivity of carbon blacks is generally correlated to their morphological structure, which can be characterized by different experimental parameters, particularly by specific surface area, measured according to the BET (Brunauer, Emmett and Teller) method, and porosity, measured by means of dibutyl phthalate (DBP) oil absorption. Usually carbon blacks having high values of BET surface area and high DBP absorption values have high conductivity values and are said to be “highly structured”. See, for example, U.S. Pat. No. 5,733,480, U.S. Pat. No. 5,476,612 and U.S. Pat. No. 6,441,984.
It is widely felt the need to increase the conductivity of polymeric materials so as to produce semiconductive elements for electric articles as described above having increased effectiveness and/or reduced thickness.
For instance, U.S. Pat. No. 4,585,578 relates to an electrically conductive plastic complex material containing as essential constituents 30 to 80 wt. percent of base plastic complex material (component A), 5 to 40 wt. percent of electrically conductive carbon black (component B) and 5 to 65 wt. percent of graphite as inorganic filler (component C), related to the total contents of the essential components. The plastic complex material may include different polymeric products, such as, inter alia, thermosetting resins, thermoplastic resins such as polyolefins, polystyrene, silicon rubbers such as SBR, butadiene rubber, polyisoprene, EP rubber, NBR or polyurethane rubbers. Preferably, the electrically conductive carbon black has a particle size in the range of 30 to 46 nm, a surface area in the range of 245 to 1000 m2/g, DBP oil absorption in the range of 160 to 340 ml/100 g. As component C, graphite can be used as it is or it may be doped for further improving electric conductivity of the conductive plastic complex material. The above compositions are said to have high electric conductivity and mechanical strength, and also a volume resistivity which is not affected by variation of temperature.
U.S. Pat. No. 5,476,612 relates to a method for preparing polymeric compositions rendered antistatic or electrically conductive by incorporating into a non-conductive matrix polymer a combination of: (A) a first finely divided conductive material, namely conductive carbon black with a BET surface area of more than 80 m2/g or an intrinsically conductive organic polymer in complexed form; and (B) a second finely divided conductive material, namely graphite or an intrinsically conductive polymer in complexed form, which is different from the material used as material A, or a metal powder; and/or (C) a finely divided non conductive material having an average particle size below 50 μm. At a given additive content in the polymer matrix the conductivity of the compound is significantly increased if a finely divided (preferred average particle size≦1 μm) conductive material A is combined with another conductive material B consisting preferably of larger particles of >0.5 μm, e.g. about 10 μm (1 to 50 μm), and/or a non-conductive material C having an average particle size <10 microns. Graphites are suitable as material B. Particularly preferred is intercalated graphite, e.g. graphite loaded with copper(III)-chloride or with nickel(III)-chloride. Further electrode graphite or natural graphite may be used. Metal are also useful as material B. As material C essentially all pigments, fillers and other non-conductive particulate materials which are non-fusible under processing conditions or materials which are insoluble in the polymer matrix and having an average particle size of about 50 microns or less may be used.
U.S. Pat. No. 5,733,480 relates to semiconductive polyolefin compositions comprised of: (a) 85 to 94 weight percent polyethylene having a density of 0.910 to 0.935 g/cm3 and melt index of 2 to 15 g/10 min; and (b) 6 to 15 weight percent of a carbon black mixture consisting essentially of: (i) 10 to 90 percent highly conductive carbon black having a particle size of 10 to 50 nm, BET surface area greater than 500 m2/g, DBP adsorption number of 200 to 600 ml/100 g and volatiles content of 2% or less; and (ii) 90 to 10 percent conductive carbon black having a particle size of 10 to 50 nm, BET surface area of 125 to 500 m2/g, DBP adsorption number of 80 to 250 ml/100 g and volatiles content of 2% or less. The above compositions are said to be readily processable so as to be extruded into films and coatings having high conductivity, good opacity and good surface quality. Furthermore, in view of the ability to use carbon black levels of 15 percent and below, the resulting films and coatings also exhibit good flexibility and mechanical properties. The above balance of properties and processability is achieved through the use of a combination of two conductive blacks of differing structure.
U.S. Pat. No. 6,441,084 relates to LLDPE (linear low density polyethylene) semi-conductive compositions having improved processability and extrudability for wire and cable applications. The semi-conductive extrusion compositions comprise: (a) 75 to 95 weight percent, based on the total weight of the composition, of a base resin comprising linear low density polyethylene having a density from 0.890 to 0.925 g/cm3 and melt index from 0.3 to 15 g/10 min; and (b) 5 to 25 weight percent, based on the total weight of the composition, of a carbon black mixture containing a major portion of a higher structure conductive carbon black and a minor proportion of a lower structure conductive carbon black. Preferably, the higher structure black has a BET surface area greater than 500 m2/g and dibutyl phthalate absorption number from 200 to 600 ml/g and the lower structure black has a BET surface area of 125 to 500 m2/g and dibutyl phthalate absorption number of 80 to 250 ml/g.
U.S. Patent Application No. 2007/0007495 relates to a composition comprising carbon black-containing polyetherester, which comprises ≦about 3.5 weight % of carbon blacks having a DBP (dibutyl phthalate oil adsorption)>about 420 cc/100 g. The composition can comprise about ≦15 weight % of carbon blacks having a DBP between about 220 cc/100 g and about 420 cc/100 g. The composition can also comprise ≦about 15 weight % of carbon blacks having a DBP between about 150 cc/100 g and about 210 cc/100 g. The reduced level of carbon blacks in the above polyetherester compositions is said to achieve the desired electric properties without unduly deteriorating the other valued melt viscosity, processing and shaped article properties.
The composition or polyetherester may be filled with about 1 to about 40 weight % of various inorganic, organic and clay fillers, which include, inter alia, graphite fibers. Such filler may improve the toughness of the composition, increase the Young's modulus, improve the dead-fold properties, improve the rigidity of the film, coating, laminate, or molded article, decrease the cost, and reduce the tendency of the film, coating, or laminate to block or self-adhere during processing or use. The carbon-black containing polyetherester can be coated or laminated onto a substrate. The coated substrates may have a variety of uses including, inter alia, semiconductive cable jacket.
EP Patent Application No. 1 052 654 A1 relates to an electric power cable having a semiconducting shield. The cable comprises one or more electric conductors, each electric conductor being surrounded by a layer comprising: (a) polyethylene; polypropylene; or mixtures thereof; (b) carbon nanotubes; (c) optionally, a conductive carbon black other than carbon nanotubes; and (d) optionally, a copolymer of acrylonitrile and butadiene, wherein the acrylonitrile is present in an amount of about 30 to about 60 percent by weight based on the weight of the copolymer or a silicone rubber. The carbon nanotubes are made of carbon and are high strength sub-micron sized fibril particles having a graphitic morphological structure and configuration. A typical carbon nanotube can be described as a tube made up of eight layers of rolled-up graphite sheets having a hollow core 0.0005 micron in diameter and an outer diameter of 0.01 micron (100 Angstroms). Their BET surface area is of about 250 m2/g; and the DBP absorption is 450 cc/100 g. When the carbon nanotubes are essentially the only carbon in the semiconducting layer composition, they can be used in amounts of about 1 to about 35 parts by weight per 100 parts by weight of component (a). When they are used together with another conductive carbon black, the weight ratio of carbon nanotubes to conductive carbon black can be about 0.1:1 to about 10:1, and the total of carbon nanotubes and other conductive carbon black can be in the range of about 5 to about 80 parts by weight per 100 parts by weight of component (a). Component (c) is optional, and can be a conventional conductive carbon black commonly used in semiconducting shields (the grades described by ASTM N550, N472, N351, N110, Ketjen blacks, and acetylene blacks). Where the carbon is essentially carbon nanotubes, interface roughness between the insulation and the semiconducting shield is said to be eliminated and the cleanliness of the semiconducting shield is said to be increased.